Whenever I'm not thrilled about one of my paintings, I'll either destroy it or (most of the time) paint over it. (I work mainly in oils.) If the painting is really good, but not one of my absolute favorites, I'll still list it on eBay. - You never know when it'll strike a chord with someone else. But, if I totally hate how something turned out, I won't sell it.
Think about it: Don't you want whatever art that gets "out there" to represent you as an artist? Personally, if I hate it, I don't want it as part of my body of work. I want only what I consider good or great to be part of my body of work. Making bad art is fine; We all have to practice. But, selling something you personally hate just doesn't feel right to me.
All the best,
-Jendria